Long‐baseline acoustic navigation for under‐ice autonomous underwater vehicle operations

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Abstract
The recent Arctic GAkkel Vents Expedition (AGAVE) to the Arctic Ocean's Gakkel Ridge (July/August 2007) aboard the Swedish icebreaker I/B Oden employed autonomous underwater vehicles (AUVs) for water‐column and ocean bottom surveys. These surveys were unique among AUV operations to date in requiring georeferenced navigation in proximity to the seafloor beneath permanent and moving ice cover. We report results for long‐baseline (LBL) acoustic navigation during autonomous under‐ice surveys near the seafloor and adaptation of the LBL concept for several typical operational situations including navigation in proximity to the ship during vehicle recoveries. Fixed seafloor transponders were free‐fall deployed from the ship for deep positioning. The ship's helicopter collected acoustic travel times from several locations to georeference the transponders' locations, subject to the availability of openings in the ice. Two shallow beacons suspended from the ship provided near‐surface spherical navigation in ship‐relative coordinates. During routine recoveries, we used this system to navigate the vehicles into open water near the ship before commanding them to surface. In cases in which a vehicle was impaired, its position was still determined acoustically through some combination of its acoustic modem, the fixed seafloor transponders, the ship‐deployed transponders, and an onboard backup relay transponder. The techniques employed included ranging adapted for a moving origin and hyperbolic navigation.